Recording paper, printable with a stylus, is made by forming an opaque layer on a colored substrate. The printing effect is achieved either by removing or by imparting a light transparency to portions of the opaque layer by contact with the stylus. Various types of recording papers are known in the art.
Those papers on which the recording is made by scraping the stylus over the opaque layer are made by applying a lacquer containing film forming components such as nitrocellulose or cellulose acetate on a black-colored paper and opacifying the coating (e.g., Japanese Published Patent Publication Nos. 34-8163 and 41-19274); or by applying a white coating containing titanium dioxide pigment or the like on a black-colored paper (e.g., Japanese Published Utility Model Publication No. 39-33446). Local scraping of the surface removes the coating and discloses the underlying black surface, the stylus being moved in response to an electrical signal.
For those papers to which transparency is to be imparted by the stylus, a light diffusion layer and a layer embodying solvent containing capsules are deposited on a colored paper substrate (Japanese Patent No. 520631). With this type of recording paper the recording is made by breaking the capsules with the stylus (moved in response to an electrical signal), resulting in releasing the solvents to permeate into the light diffusion layer. Thus, the light diffusion layer is made locally transparent and the underlying color of the colored substrate is disclosed.
Among the former processes, the recording paper using lacquers employ low boiling point solvents such as ketones (for example, acetone), alcohols (for example, methanol) or the like to dissolve the film forming components such as nitrocellulose or cellulose acetate. Obviously, the risk of fire and explosion are great hazards in the coating process. These components are also detrimental to the working environment as the solvents evaporate into the air. Another difficulty inherent in this process is that the lacquer coating must be subjected to a prolonged period of drying at a low temperature or a mild heating under an optimum humidity to obtain the required whitening effect. This situation is quite adverse to the improvement of productivity. Also, the recording paper that is prepared by applying a white coating has a poor appearance due to a resultant coarse surface and is deficient in print definition and printability.
The recording paper made by the latter process (i.e. requiring definite breakage of capsules by a stylus) involves the problem of slow printing speed and subsequent poor storage stability owing to the discoloration of the printed sheet.
U.S. Pat. No. 2,739,909 discloses a process by which a lacquer mixture is prepared by first introducing a W/O (water-in-oil) type emulsifier into a resinous organic solution in which PMMA is dissolved. Thereafter, water is added under agitation to disperse micron size water droplets therein. The recording paper is subsequently prepared by applying the mixture on the colored substrate which is then dried. According to the U.S. Patent, the solvents first evaporate in the drying process, and a coating film of PMMA which embodies the dispersed water droplets therein is formed. Then, when water in the droplets evaporates, the white opaque layer is formed in which voids are left in the polymer coating film.
In this case again, the polymer solution contains a large amount of solvent and does not contribute to solving the problem of fire and explosion hazards and deterioration of the working environment similar to the process using lacquers.